The purification in the liquid phase of organic feedstocks such as hydrocarbons containing sulfur compound impurities by the selective adsorption of the impurity compounds on molecular sieve adsorbents is well know in the art. For example, the liquid phase purification of petroleum-derived full range hydrocarbon feedstocks containing carbonyl sulfide is disclosed in U.S. Pat. No. 3,654,144 issued to J. J. Collins, Apr. 4, 1972. Therein the process comprises passing the sour hydrocarbon feed in the liquid phase through a fixed bed of a bivalent metal cation form of zeolite A to selectively adsorb the COS. The periodic regeneration of the adsorbent bed is accomplished in the vapor phase using a non-sorbable purge gas such as nitrogen for displacement of the void space liquid as well as desorption of the COS impurity. This procedure is typical of the prior art processes insofar as regeneration is concerned. The entire volume of the gas stream used for displacement and purge desorption is isolated from the system and is largely a waste material. Equally disadvantageous economically, is the fact that the full bed void space volume of vapor phase hydrocarbon remaining in the bed along with a significant quantity of "sponged" liquid hydrocarbon and coadsorbed hydrocarbon becomes admixed with the purge gas and/or the desorbed COS impurity constituent and is either not recovered as product or must be reclaimed by a further purification procedure.
In commonly assigned U.S. application Ser. No. 155,125, filed Feb. 11, 1988 now U.S. Pat. No. 4,899,016 by K. R. Clark and P. Richman there is disclosed an improved process for the partial recovery of "sponged" liquid hydrocarbon. In accordance with this process the adsorbent bed regeneration procedure involves the use of a portion of the purified feedstock in the vapor phase to countercurrently remove the liquid held in the bed void space and a relatively non-sorbable purge gas to sequentially displace the purified feedstock vapor from the bed using plug-flow followed by a displacement of the non-sorbable purge gas from the bed void space using another portion of the purified feedstock in the vapor phase, and finally cooling and refilling the bed in a direction cocurrent with the flow of feedstock into the bed during the adsorption purification stage with a portion of the purified feedstock in the liquid phase.
A particularly difficult problem arises in processes for the purification of propane and other light hydrocarbons containing carbonyl sulfide in which liquid phase adsorption on molecular sieve adsorbents is employed to selectively adsorb the COS. It has been found that as much as 26 weight per cent of the light hydrocarbon treated is coadsorbed with the COS. Since inert gas purging of the adsorbent bed ordinarily desorbs COS and light hydrocarbon simultaneously, the desorbed hydrocarbon effluent from the bed is too highly contaminated with COS to be commercially useful.